There is a wide range of ternary and quaternary II-VI and III-V semiconductor compounds, which are difficult to grow into high quality single crystals from the melt. Principally, there are these four reasons: very high melting points, non-congruent melting, decomposition or evaporation on melting or having a melting point above the desirable crystallographic phase.
For example, totally molten CZT requires a temperature in excess of 1100° C. (above its liquidus temperature). A hot-walled or a vessel under high inert gas pressure is thus required to prevent the molten CZT from decomposing/subliming to the cooler locations. Such pressures may exceed 100 atmospheres, requiring expensive reactors.
Several growth methods have been used for the growth of bulk CZT. These include Horizontal Bridgman (HB) and Vertical Bridgman (VB) or Vertical Gradient Freeze (VGF) in sealed systems; High Pressure Vertical Bridgman (HPVB) in unsealed ampoules; the Traveling Solvent Method (TSM) and the Traveling Heater Method (THM).
Limitations of Current Art
There are many limitations and problems with Bridgman-type melt growth methods. Varying gradients and uncontrolled temperature fluctuations at the crystal growth interface serve to induce crystal defects and inhomogeneities. Processes requiring fused sealed ampoules or “closed tubes” incur the additional cost burden of single-use ampoules. Specifically for CZT, the Bridgman VGF method produces large axial variations in Zn concentration, because of the non-congruent melting property. Additionally, the relatively long temperature ramps and slow growth rate again increase the cost of forming CZT by this method.
The reaction to compound CZT from elemental Cd, Zn and Te is highly exothermic and unstable. It can occur unpredictably with explosive force. The consequence of such explosions may include damage to the apparatus, loss of expensive reagents, distribution of toxic materials into the environment and risks to those personnel in the vicinity. To mitigate against these effects expensive explosion-proof apparatus and facilities are necessary.
The three constituents, Cd, Zn and Te, each have different melting points below the melting point of CZT. As the temperature reaches the range ˜600-900 C unreacted liquid Te, liquid Cd and liquid Zn attempt to coexist with solid CZT already formed through the reaction of the components. As the temperature rises various reactions continue to occur between liquids and solids and between liquids and liquids to form other liquid or solid intermediaries or the desired CZT end product. The mixture is highly heterogeneous in terms of the solid, liquid and vapour phases and in terms of the temperature distribution in the reaction vessel. The rate of reaction is influenced both by the chance contact of reagents and intermediate compounds and by the extent of the heat generated—raising the local, and therefore the average, temperature and further accelerating the reaction. Very high Cd vapour pressures can occur, for example, if unreacted elemental Cd is suddenly heated to a high temperature.
The high temperatures for long periods typical in melt growth processes can cause oxides to build up on the boule surface, resulting in the boule adhering to the walls of the containment vessel, such as a quartz ampoule. This may cause difficulty in releasing the boule from the vessel. Additionally. due to the high temperatures of ˜1100° C. at peak and for an extended duration in conventional CZT compounding processes contaminants can leach from the quartz ampoule.
Typically, sealed ampoules used in VGF compounding processes require a portion of the sealed ampoule to be cut off to extract the boule, causing waste and possibly precluding reuse of the ampoule. The VGF ampoules must be evacuated to high vacuum and use a quartz/quartz fused seal. If a slow leak occurs during the subsequent heating cycle the ampoule is highly probable of rupturing.
Many problems of compounding additional materials are specific to each material, and its intended subsequent use. Important compounds are ZnTe, CdTe, and presaturated CZT solvent used to grow large-grained CZT crystals. Each of these is briefly summarized. The greater the quality of polycrystalline compounded material the greater the chance of subsequently re-crystallizing it into excellent single crystal material.
An example of a compound requiring a very high temperature is ZnTe. The melting point of zinc telluride is ˜1239° C., so compounding ZnTe via direct melting would require expensive coated quartz ampoules or higher temperature crucibles, which are prone to contaminate their contents with heavy metals.
CdTe is an important compound for detectors and energy conversion devices, and has a melting point above the desired crystallographic phase. It also decomposes/evaporates on melting. Sealed, single use, ampoules are necessary to contain the pressures generated. Excess Te as a solvent is frequently used to reduce the reaction temperature but this slows the rate of reaction and triggers other problems.
A Te rich CZT solvent formulation can be used for growth of CZT crystals. Preparing this formulation having excess Te, by conventional methods, results in inhomogeneity and improper stoichoimetry, for example saturating the Te solvent with standard feed that has a Cd/Zn ratio of 9:1 has two disadvantages. Firstly, it means using more expensive synthesized feed rather than the elements, and secondly the Cd:Zn ratio is not the equilibrium value and causes instabilities and Zn inhomogeneities in the subsequent process.
Cost issues due to slow processes and high cost reactor equipment are common to the previous conventional compounding techniques. Contamination from reactor vessel leachate and oxides are also a common problem.
Techniques used to stabilize the compounding of other highly reactive materials in other industries frequently are inapplicable with respect to semiconductors, especially CZT, CdTe and ZnTe.